Astronomers, using the Wide Field Planetary Camera 2 on board NASA's
Hubble Space Telescope in October and November 1997 and April 1999,
imaged the Bubble Nebula (NGC 7635) with unprecedented clarity.
For the first time, they are able to understand the geometry and
dynamics of this very complicated system. Earlier pictures taken of
the nebula with the Wide Field Planetary Camera 1 left many issues
unanswered, as the data could not be fully calibrated for scientific
use. In addition, those data never imaged the enigmatic inner
structure presented here.

The remarkably spherical "Bubble" marks the boundary between an
intense wind of particles from the star and the more quiescent
interior of the nebula. The central star of the nebula is 40
times more massive than the Sun and is responsible for a stellar
wind moving at 2,000 kilometers per second (4 million miles per hour
or 7 million kilometers per hour) which propels particles off the
surface of the star. The bubble surface actually marks the leading
edge of this wind's gust front, which is slowing as it plows into
the denser surrounding material. The surface of the bubble is not
uniform because as the shell expands outward it encounters regions
of the cold gas, which are of different density and therefore arrest
the expansion by differing amounts, resulting in the rippled
appearance. It is this gradient of background material that the
wind is encountering that places the central star off center in the
bubble. There is more material to the northeast of the nebula than
to the southwest, so that the wind progresses less in that direction,
offsetting the central star from the geometric center of the bubble.
At a distance of 7,100 light-years from Earth, the Bubble Nebula
is located in the constellation Cassiopeia and has a diameter of
6 light-years.

To the right of the central star is a ridge of much denser gas. The
lower left portion of this ridge is closest to the star and so is
brightest. It is experiencing the most intense ultraviolet radiation
as well as the strong wind and is therefore being photoevaporated
the fastest. The ridge forms a V-shape in the image, with two
segments that are aligned at the brightest edge. The upper of these
two segments is viewed quite obliquely as it trails off into the back
of the nebula. The lower segment comes both toward the observer and
off to the side. This lower ridge appears to lie within the sphere
described by the bubble but is not actually "inside" the shocked
region of gas. Instead it is being pushed up against the bubble like
a hand being pushed against the outside of a party balloon. While
the edge of the hand appears to be inside the balloon, it is not.
As the bubble moves up but not through the ridge, bright blue arcs
form where the supersonic wind strikes the ridge to form an apparent
series of nested shock fronts.

The region between the star and ridge reveals several loops and arcs
which have never been seen before. The high resolution capabilities
of Hubble make it possible to examine these features in detail in a
way that is not possible from the ground. The origin of this
bubble-within-a-bubble" is unknown at this time. It may be due to a
collision of two distinct winds. The stellar wind may be colliding
with material streaming off the ridge as it is photoevaporated by the
star's radiation.

Located at the top of the picture are dense clumps or fingers of
molecular gas which have not yet encountered the expanding shell.
These structures are similar in form to the columns in the Eagle
Nebula, except that they are not being eroded as energetically as
they are in that nebula. As in the Eagle, the clumps are seen to
emit light because they are being illuminated by the strong
ultraviolet radiation from the central star, which travels much
faster than the shell and has reached the outer knots long before
the expanding rim will.